The present invention relates to highly purified titanium material suitable for target material used for forming Ti wiring network on a semiconductor device surface, a method for preparation of it, and a sputtering target using it.
Wiring network having a complicated pattern is formed on a surface of various kinds of semiconductor devices with conductive metallic materials according to a purpose of use. The wiring network is formed by forming a conductive metallic film made of Al, Au or the like by sputtering process and applying a certain etching treatment of the film for patterning.
Currently, the wiring is required to have a narrow width and a thin thickness as the semiconductor device is highly integrated. But high minute wiring network causes delay of signals due to the wiring resistance of used wiring material, or burn-out of wiring due to resistance heating during operation of the device because of using a material having a low melting point. Therefore wiring material not to need a drastic alteration of any forming processes of LSI, VLSI, and ULSI, and having a high melting point is demanded. Ti attracts attention as one of the materials for that purpose, as well as Mo, W and Ta.
When Ti is used for wiring network of a semiconductor device, usually a film made of Ti is formed by sputtering process as described above. Therefore, a sputtering target made of Ti material is required to be prepared and the Ti target must have high purity. When the Ti target contains oxygen as an impurity, increased electric resistance of the film prepared causes a problem of delay or a trouble of burn-out of wiring network. A heavy metal such as Fe, Ni or Cr makes a factor causing leakage at the joint of the film interface. An alkali metal such as Na or K readily sets Si free and degrades the characteristics of the device.
Besides the high purity mentioned above, the Ti target forming wiring network of, for example, VLSI is highly required to be uniform without any cracks or creases externally or internally, and to uniformly discharge the thermal energy produced during sputtering.
This is because the film thickness and the uniformity of inside composition of a film formed by sputtering, besides effects of an impurity, are becoming more important as high integration requires the wiring to be more minute. When scratches exist on the target surface or inside, flight of sputtering particles falls into disorder at that place, and the uniformity of bonding condition on a substrate is lowered, causing varying electric resistances or burn-out. When thermal energy applied to the target during sputtering is not uniformly discharged to the side of a backing plate, uniform bonding condition is deteriorated because of heterogeneity of temperature distribution of the target.
The above-mentioned Ti target is generally prepared by the method described below.
Crude Ti material is prepared by one of the following three methods. First method is called Kroll method or Hunter method which is to thermally reduce a Ti compound such as TiCl4 with an active metal such as Na or Mg. Second is called Iodide method which is to thermally decompose a titanium compound such as TiI4. And third is a method for molten salt electrolysis in salt such as NaCl or KCl. Because the crude Ti material prepared as above is in the shape of sponge, crystal or a needle, it is generally arc-melted in a vacuum of about 10xe2x88x922 Torr to 10xe2x88x923 Torr to be made into an ingot, which is then shaped into a target form for use.
Because the purity of the Ti target prepared by the prior methods mentioned above is about 2N to 3N, it can be used as a sputtering target for 64 Kbits but not for 256 Kbits, 1 Mbit, 4 Mbits or higher because of low purity as a target material for wiring or barrier material.
Besides, the Ti target prepared by the prior methods is insufficient in view of the surface and inside conditions mentioned above. To prevent any surface or inside scratches of the Ti target from causing a change of electric resistance or burn-out, processability shall be raised. On the other hand, to prevent contamination during preparation, highly purified Ti is shaped into a certain form by cold working. The processability of the Ti material by the prior methods is not enough to achieve sufficient homogeneity. Besides, thermal energy is insufficiently discharged. In short, the prior methods do not provide a Ti material having high thermal conductivity sufficient to uniformly discharge the thermal energy which is applied to a target during sputtering, to the side of a backing plate.
An object of the present invention is to provide a highly purified Ti material having sufficient purity, processability and thermal conductivity required for a Ti target when forming a wiring layer or a barrier layer of LSI by sputtering process.
Another object of the present invention is to provide a method for preparing a highly purified Ti material capable of preparing a highly purified Ti material having high purity, excellent processability and thermal conductivity required for a Ti target when forming a wiring layer or a barrier layer of LSI by sputtering process.
And a further object of the present invention is to provide a sputtering target capable of forming a uniform Ti film including only a few impurities which have an adverse effect on the function of a device.
The highly purified Ti material of the present invention has an oxygen content of not more than 350 ppm, Fe, Ni and Cr contents of not more than 15 ppm each, Na and K contents of not more than 0.5 ppm each, a reduction of area as a material characteristic of not less than 70%, and a thermal conductivity of not less than 16 W/m K. Besides, a sputtering target of the present invention is made by shaping the above highly purified Ti material into a desired form.
First method of preparation of the highly purified Ti material of the present invention consists of a step of sieving crude Ti particles to classify them into each type of particle diameters according to contents of impurities, a step of sorting them having the desired particle diameters from the above sieved crude Ti particles as one type or a mixture of two or more types, and melting by using an electron beam.
Second method consists of a step of acid-treating the crude Ti particles to remove a contaminated layer on the crude Ti particle surfaces and a step of melting the acid-treated crude Ti particles by using an electron beam.
The highly purified Ti material having good processability and thermal conductivity of the present invention has been achieved by finding out the facts that the crude Ti particles prepared by molten salt electrolysis or Iodide method have different purities, and particularly oxygen contents depending on the particle diameter, that the impurities contained in the crude Ti particles exist as concentrated in the surface layer, and that a thermal conductivity of not less than 16 W/m K and a reduction of area of not less than 70%, as well as high purity, which heretofore could hardly be obtained by the prior arts can be obtained by using sieving and acid-treating methods.
The highly purified Ti material of the present invention can be prepared for example as follows.
First, crude Ti particles which make a starting material of the present invention are prepared by molten salt electrolysis for example. As the Ti material used for molten salt electrolysis, for example, sponge Ti, preferably one including less U and Th contents is used. KCl-NaCl is preferably used for electrolytic bathing (electrolyte), and an electrolytic temperature of 730 to 755xc2x0 C. and a voltage of 6.0 to 8.0 V are preferable. The Ti particles prepared by molten salt electrolysis usually has high contents of Na and K and relatively less heavy metals such as Fe and Ni or oxygen. The crude Ti particles as a starting material of the present invention are not limited to those prepared by molten salt electrolysis, but include those which can have a desired purity by melting with an electron beam (hereinafter called EB melting) to be mentioned below. For example, crystal Ti prepared by Iodide method or sponge Ti then acid-treated can be used.
In the present invention, the above crude Ti particles (for example, needle-shaped Ti particles) are subjected to at least one selected from the following two kinds of treatments. (1) The crude Ti particles are sieved with a nonmetallic sieve such as of nylon for preventing contamination from outside, to be classified based on each particle diameter (outer diameter of Ti particles and the same is applied hereinafter). (2) The crude Ti particles are acid-treated to remove a contaminated layer existing on the surface.
Impurities contained in the crude Ti particles prepared by molten salt electrolysis exist as concentrated on the surface layer. Therefore, the particle diameters are classified as in the above (1) to select needle-shaped Ti particles having bigger particle diameters, so that the specific surface area is decreased and the content of impurities is proportionally decreased Selecting the particle diameters to be used decreases particularly oxygen content and at the same time provides the final product with a thermal conductivity of not less than 16 W/m K and a reduction area of not less than 70%.
Forcibly removing the surface contaminated layer as in the above (2) decreases the content of impurities and at the same time provides the final product with a thermal conductivity of not less than 16 W/m K and a reduction of area of not less than 70%. The acid treatment is advantageous for removing the contaminated layer, particularly heavy metals such as Fe, Ni and Cr.
The same is also applied to Ti particles prepared by Iodide method.
In the method of sieving as in the above (1), it is preferable to selectively use the Ti particles having a particle diameter of not less than 1 mm without many impurities, particularly those with a particle diameter of not less than 2 mm. The particle diameters of the crude Ti particles are not limited to not less than 1 mm, but Ti particles having smaller particle diameters can be used together within an allowable range of impurity contents in the Ti material of the present invention because the impurities are present in the Ti particles in proportion to the particle diameters. In this case, it is preferable to use not less than 90% of the crude Ti particles having a particle diameter of not less than 1 mm. Minute Ti particles having a particle diameter of not more than 0.5 mm are preferably not to be used because vacuum may be unstable during EB melting.
The acid treatment as in the above (2) is preferably operated by acid-treating in an inert atmosphere such as of argon gas, washing with purified water, and drying to prevent re-contamination of the surface layer (especially by oxygen). The acid solution to be used is, for example, hydrofluoric acid, hydrochloric acid, nitric acid, a mixture of hydrofluoric acid and hydrochloric acid, and a mixture of nitric acid and hydrochloric acid. To remove only the surface layer, a mixture of hydrochloric acid, fluoric acid and water in a ratio of 0.8 to 1.2:1.8 to 2.2:36 to 38 by volume is preferably used.
The content of impurities can be decreased by sieving according to the above-mentioned (1) to select the Ti particles having a greater particle diameter such as not less than 1 mm and treating them to remove their surface contaminated layers. Sieving after the acid treatment results in the same.
Thus, the impurities remaining in the crude Ti particles are further decreased by either one of the above-mentioned treatment (1) and (2), and Na and K are finally removed by EB melting. The EB melting can remove particularly Na and K effectively.
Generally, to effect the EB melting, the crude Ti particles may be compressed by press molding to be solidified and used as an electrode for EB melting. In this case, because re-contamination might be caused by tools or deformation when molded, the present invention preferably puts the crude Ti particles (needle-shaped Ti particles) as they are into a vibrating granulator in vacuum and EB-melts to prevent the re-contamination.
The crude Ti particles are EB-melted in an EB melting furnace which is controlled to an inside vacuum of 5xc3x9710xe2x88x925 mbar, preferably not more than 2xc3x9710xe2x88x925 mbar, preventing a diffusion pump oil from entering into the furnace by a freon baffle. Operating conditions of the EB melting are not particularly limited but are required to select a melting speed considering a degree of removing Na and K, and contamination by absorption of oxygen. For example, it is desirably 1.75 to 2.3 kg/hour.
In the above step, remarkably improved conductance of vacuum exhaust solves a disadvantage that the oxygen content increases when arc melting is employed. By sieving to remove minute particles, stable vacuum is attained at the low vacuum side in EB melting and the EB-casted material prepared has its oxygen content suppressed to not more than 350 ppm, and other impurities are to be decreased but not increased.
The Ti material prepared by the above-mentioned procedure has a high purity of an oxygen content of not more than 350 ppm, Fe, Ni, and Cr contents of not more than 15 ppm each, Na and K contents of not more than 0.5 ppm each and U and Th contents of not more than 1 ppb each, the high processability of a reduction of area as a material characteristic of not less than 70%, and a high thermal conductivity of not less than 16 W/m K. Besides, the Ti material prepared under different conditions selected has a high purity of an oxygen content of not more than 250 ppm, Fe, Ni, and Cr contents of not more than 10 ppm each and Na and K contents of not more than 0.1 ppm each, and a high processability of a reduction of area as a material characteristic of not less than 80%, and a high thermal conductivity of 17 W/m K. Additionally, the Ti material obtained satisfies a high purity of an oxygen content of not more than 200 ppm, Fe, Ni and Cr contents of not more than 5 ppm each and Na and K contents of not more than 0.05 ppm each, the high processability of a reduction of area as a material characteristic of not less than 85%, and the high thermal conductivity of not less than 18 W/m K.
To prepare the sputtering target of the present invention, first of all, the highly purified Ti material prepared by the above-mentioned method is cold-forged into a desirable form without re-contamination. The cold forging shall be operated in a cold condition (at about room temperature) while preventing re-contamination by absorbing gases considering the characteristics of the Ti material easily absorbing any gases. After that, the sputtering target of the present invention is prepared by shaping the cold-forged Ti material into a certain target form by machining.
The cold processability of the Ti material depends on the above-mentioned reduction of area. Because the Ti material having a reduction of area of not less than 70% can satisfy the cold processability in forming it into a sputtering target, the sputtering target not having heterogeneous part such as scratches and creases inside or on the surface of it can be formed to have a certain form. The reduction of area is preferably not less than 80%, particularly not less than 85%. Sputtering using the Ti target controls the irregular flight of sputtering particles caused by the heterogeneous part and provides a film having more uniform thickness and inside structure.
The above-mentioned reduction area shall have a value measured according to JIS Z 2241. With a specific measuring method, a sample is set in a tensile tester according to JIS B 7721 and pulled in an axial direction, and a reduction area "psgr" (%) is calculated form the area A of a fracture plane and the initial cross section A0 by the following formula (I).
"psgr"=(A0xe2x88x92A)/A0xc3x97100xe2x80x83xe2x80x83(I)
Using the high thermal conductive Ti material having a thermal conductivity of not less than 16 W/m K as a sputtering target permits the thermal energy applied to the target during sputtering to be uniformly discharged to the side of a backing plate. The thermal conductivity is preferably not less than 17 W/m K, particularly not less than 18 W/m K. Because sputtering can be operated in a uniform heating condition in a whole target, more homogeneous film can be prepared.
The above-mentioned thermal conductivity is measured by Flash method. Specifically, the method consists of radiating a pulse light onto a sample surface uniformly by laser for instance, measuring the increased temperature on the back surface of the sample to obtain a thermal diffusivity xcex1, and calculating the thermal conductivity xcex from the thermal diffusivity xcex1 as follows:
xe2x80x83xcex1=1.37L2/(xcfx802xc2x7txc2xd)xe2x80x83xe2x80x83(II)
where,
L=a thickness of the sample, and
txc2xd=a time until the temperature on the back surface of the samble reaches a half of maximum, and
xcex=xcex1xc2x7Cpxc2x7xcfx81xe2x80x83xe2x80x83(III)
where,
Cp=a specific heat capacity, and
xcfx81=a density of the sample.
The present invention provides a Ti material having a high purity, high processability and high thermal conductivity by an easy method. Using the Ti material as a target material for sputtering process permits high minute wiring on LSI or ULSI for instance and a film for barrier to be formed uniformly and good reproducibility, and suppresses the content of undesirable impurity elements to significantly small.